1. Field of the Invention
The present invention relates to a DC-DC converter and, more particularly, the present invention relates to an insulated DC-DC converter used in such a way that the primary side and the secondary side are insulated from each other.
2. Description of the Related Art
In an insulated DC-DC converter, it is common practice for a photocoupler to be used for the feedback of a signal from the secondary side to the primary side (the output voltage is detected, and based on this voltage, the switching of a switching element is controlled via a switching control circuit of the primary side). However, several problems arise in that, since, in the photocoupler, a phase delay becomes larger and responsiveness becomes poor with respect to a high-frequency signal, if the switching frequency is increased, oscillation is likely to occur, and responsiveness with respect to load variations becomes poor.
Accordingly, in order to solve these problems, a DC-DC converter described in Japanese Unexamined Patent Application Publication No. 8-331844 has been proposed. This DC-DC converter is such that responsiveness is improved by making corrections by using a primary-side signal with a small phase delay. That is, as shown in FIG. 5, the transformer is provided with a primary coil n1, a secondary coil n2, and an auxiliary coil ns. An input voltage source 110 rectifies and smoothes the AC current from, for example, the commercial AC power source in order to generate a DC voltage Vin. The input voltage source 110 is connected to the primary coil n1 and is turned on/off by the switching element Q of a transistor, etc. Since a switching voltage signal is induced in the secondary coil n2, an output voltage Vout, which is transformed into a direct current by the rectifying and smoothing circuit of a diode D1 and an output capacitor 113, is supplied to a load 120.
An error amplifier 130 compares the output voltage Vout with the reference voltage, and outputs an error signal corresponding to this error voltage. A photocoupler 140 insulates the primary side of the DC-DC converter 100 from the secondary side thereof, wherein the error signal output from the error amplifier 130 is transmitted to the primary side. A drive circuit 150 inputs an error signal transmitted from the photocoupler 140, and applies, to the switching element Q, a switching control signal in a direction in which the output voltage Vout and the reference voltage match each other. An auxiliary power-supply circuit 160 generates a voltage for operating the drive circuit 150 in such a way that the switching signal induced in the auxiliary coil ns is transformed into a direct current by the rectifying and smoothing circuit of a diode D2 and a capacitor 114 and an auxiliary power voltage is generated.
Voltage-dividing resistors 116 and 117 divide the output voltage of the auxiliary power-supply circuit 160, and generate a signal that is proportional to the output voltage of the auxiliary power-supply circuit 160. A coupling capacitor 115 superimposes, in an AC manner, the output voltage signal sent from the voltage-dividing resistors 116 and 117 onto the error voltage signal transmitted to the drive circuit 150 from the photocoupler 140.
In this manner, the voltage induced in the auxiliary coil ns provided in the transformer is rectified and smoothed by the auxiliary power-supply circuit 160, this voltage is detected by using the output voltage detection elements, namely, the voltage-dividing resistors 116 and 117, by resistance division, and the AC components contained in this detected signal are superimposed in an AC manner onto the output of the photocoupler 140 via the coupling device (the coupling capacitor 115). Thus, stability with respect to oscillation, etc., is improved.
However, in a conventional DC-DC converter 100, since the voltage induced in the auxiliary coil ns is detected by using the output voltage detection elements 116 and 117 by resistance division, there are problems in that the level of the signal superimposed in an AC manner onto the output of the photocoupler 140 becomes extremely low. In particular, the serious problems are that the flexibility of constant design is low, and there are a large number of limitations on the aspects of phase correction, starting characteristics, and short-circuit protection.
For the voltage-dividing resistor 116 of the output voltage detection elements, usually, a resistor having a large resistance value (10 kΩ or more) is used to prevent a decrease in the efficiency (an increase in the loss). Here, to ensure a sufficient amount of phase correction, it is necessary to decrease the impedance of the series circuit of the coupling capacitor 115 and the voltage-dividing resistor 116. However, since the resistance R of the voltage-dividing resistor 116 is large, if attempts are made to decrease the impedance, it is necessary to increase the electrostatic capacitance C of the coupling capacitor 115.
When the DC-DC converter 100 is to be started, the coupling capacitor 115 is charged by the time constant CR represented by the product of the electrostatic capacitance C thereof and the resistance R of the voltage-dividing resistor 116. However, since the time constant CR is large, the charging time is increased. The rise waveform of the output voltage Vout at the starting time becomes less sharp, and the starting time tr1 is increased (see FIG. 6).
The DC-DC converter is usually provided with a short-circuit protection circuit including a diode, a transistor, etc. This short-circuit protection circuit is so designed that, in order to decrease stress imposed on a diode and a transistor during the short-circuited time, a mask time becomes as short as possible so that the short-circuit protection circuit does not malfunction during the starting time. On the other hand, in order that the short-circuit protection circuit does not malfunction during the starting time, it is necessary to set the mask time to be greater than or equal to the starting time tr1. Therefore, when the starting time tr1 is long as shown in FIG. 6, the mask time cannot be decreased, and thus, for diodes and transistors, there occurs the necessity to use expensive and large types which can withstand a large stress during the short-circuited time.